A thermal imager detects infrared radiation emitted from an object or scene within its field of view. It converts the infrared radiation emitted into electrical signals that are displayed on a screen. Thermal imagers convert infrared radiation into visible light. Thermal imagers typically operate at wavelengths from 7,500 to 14,000 nm or 7.5 to 14 μm. Two objects or areas of a scene at the same temperature are displayed on a screen using the same colour.
Some thermal imagers can calculate the temperature of an object, taking into account the objects emissivity, ambient humidity, reflected temperature and the distance between the object and the thermal imager.
Thermal imagers are commonly used to inspect electrical equipment. The thermal imager is used to measure infrared radiation or heat produced by the electrical equipment and converts the measurements into a visible image. Regular inspections of the electrical equipment allow an operator to compare successive readings and thereby detect when the thermal images produced vary over time. An increased thermal reading indicates the particular area of the electrical equipment is heating up and is therefore prone to failure. A thermal imager can therefore be used to provide an early warning system for component failure and help organise a service regime that concentrates on components that are failing and need replacement.
Thermal imagers often use a bolometer, specifically a microbolometer, as a detector to convert the infrared radiation emitted by an object into visible light. The microbolometer measures the energy on incident electromagnetic radiation and is typically a grid of heat sensors comprising vanadium oxide or amorphous silicon. On top of these heat sensors is a grid of silicon. Infrared radiation that strikes the vanadium oxide changes its electrical resistance and this change is measured and used to generate the visual image.
Microbolometers do not require cooling and so are commonly used as the detector in handheld thermal imagers when it would be difficult to cool the detector using, for example, liquid nitrogen.
Some thermal imagers are required to be used in hazardous and/or explosive atmospheres. Special consideration must be given to the arrangement of the component parts of such thermal imagers so that they are intrinsically safe.